In wireless communication systems that use radio frequency (RF) transmissions, an RF signal from a transmitter may reach a receiver via a number of propagation paths or channels. To provide diversity against harmful path effects and improve performance, multiple transmit and receive antennas are used. Propagation paths between the transmit and receive antennas are independent when a transmission on one path is not formed as a linear combination of the transmissions on the other paths.
A multiple-input-multiple-output (MIMO) communication system employs multiple transmit antennas and multiple receive antennas for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels, wherein each channel is a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
In spatial multiplexing, there are different data streams/paths in the same channel wherein a different transmission antenna is used for each data stream. In MIMO systems, in order to realize advantages of spatial multiplexing for high spectral efficiency, the wireless channels need to be multipath-rich and less correlated.
To increase spectral efficiency in MIMO systems, some conventional approaches use a transmitter that employs a spatial multiplexing scheme to send signals to the receiver. This includes finding a pair of linear transformations at transmitter precoder and receiver decoder in the space-domain such that maximum spectral efficiency is reached. However, although throughput (spectral efficiency) is increased, communication link robustness may be reduced. Examples of such conventional approaches for linear transmitter precoder and receiver decoder optimization in the space-domain which only increase spectral efficiency without increasing system link robustness are discussed in one or more of the following five papers, incorporated herein by reference: (1) G. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J., vol. 1, no. 2, 1996, (2) L. Zheng and D. Tse, “Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels,” IEEE Trans. Info. Theory, vol. 49, May 2003, (3) F. R. Farrokhi, G. J. Foschini, A. Lozano, and R. A. Valenzuela, “Link-optimal space-time processing with multiple transmit and receive antennas,” IEEE Comm. Letters, vol. 5, March 2001, (4) H. Sampath, P. Stoica, and A. Paulraj, “Generalized linear procoder and decoder design for MIMO channels using the weighted MMSE criterion,” IEEE Trans. Comm., vol. 49, December 2001, and (5) A. Scaglione, P. Stoica, S. Barbarossa, G. B. Giannakis, and H. Sampath, “Optimal design for space-time linear precoders and decoders,” IEEE Trans. Signal Processing, vol. 50, May 2002.
Other conventional approaches have incorporated space-time diversity to increase communication link robustness by incorporating time-domain delays at each transmit communication path. However, although communication link robustness is increased, throughput performance is decreased. Examples of such conventional approaches which include switching between time diversity and spatial multiplexing to accommodate link robustness and spectral efficiency, without joint optimization, are discussed in one or more of the following two papers, incorporated herein by reference: (1) D. Gesbert, L. Haumonte, H. Bolcskei, R. Krishnamoorthy, A. Paulraj, “Technologies and performance for non-line-of-sight broadband wireless access networks,” IEEE Communications Magazine, April 2002, and (2) L. Zheng and D. Tse, “Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels,” IEEE Trans. Info. Theory, vol. 49, May 2003.
There is, therefore, a need for a method and system that jointly increases the MIMO system performance by introducing time-domain variable delay at each path in the transmitter along with space-domain precoder and decoder optimization.